Seat occupant detection systems are commonly in connection with air bags and other pyrotechnically deployed restraints as a means of judging whether, and how forcefully, to deploy the restraint. It has been shown that the seated weight of the occupant in combination with the weight distribution over the seating area can be used to reliably classify the occupant for purposes of restraint deployment. For example, normal energy deployment can be enabled for adults, reduced energy deployment can be enabled for a seated child, and deployment can be disabled entirely for a child seat or infant seat.
Of the various ways that have been devised for determining the seated weight and weight distribution of an occupant, perhaps the most simple and cost-effective involves installing a multi-sensor pad in or under the bottom foam cushion of the seat. For example, the U.S. Pat. No. 5,474,327 to Schousek and U.S. Pat. No. 5,732,375 to Cashler disclose the use of sensor pads comprising a plurality of variable resistance pressure sensors. A somewhat different approach is found in the U.S. Pat. No. 6,578,871 to Gray et al., where the sensor pad comprises a plurality of fluid-filled chambers corresponding to different areas of the seat and sensors for detecting the fluid pressure in each of the chambers. Finally, the U.S. Pat. No. 6,927,678 to Fultz et al. discloses a capacitive sensor pad defined by a fluid-filled seat bladder with upper and lower metalized films adjoining the major surfaces of the bladder; the bladder and its fluid form a dielectric that locally varies in thickness as a function of the force applied to the seat by an occupant. In Fultz et al., force distribution on the seat is sensed by dividing the upper (or lower) metalized film into a plurality of individual metalized segments to define an array of capacitive sensing elements with respect to the lower (or upper) metalized film.
While the capacitive sensing approach described in the aforementioned U.S. Pat. No. 6,927,678 to Fultz et al. can be very cost effective to manufacture and install, the number of sensing elements is limited by electrical connection costs. As a result, the overall resolution of the sensor pad is similarly limited and the development effort to determine optimal sensor placement for a given seat configuration can be considerable. Also, when the seat has a metallic cushion support pan, parasitic coupling between the lower metalized sensor film and the seat pan can introduce error in the capacitance measurements. Accordingly, it is desired to increase the number of sensing elements in a capacitive seat sensor array and reduce its susceptibility to measurement errors without increasing its cost of manufacture and installation.